Semiconductor device with a trench isolation and method of manufacturing trenches in a semiconductor body

Abstract

 As a first step a novel low cost integration method for a plurality of deep isolation trenches on the same chip is provided. The trenches have an additional n-type or p-type doped region surrounding the trench - silicon interface. Providing such variations of doping the trench interface is achieved by using implantation masking layers or doped glass films structured by a simple resist mask. By simple layout variation of the top dimension of the trench various trench depths at the same time can be ensured. Using this method, wider trenches will be deeper and smaller trenches will be shallower.

Description

[0001] The invention relates to the integration of high voltage and preferably small signal transistors into integrated circuits (IC's) on monocrystalline semiconductor wafers. Isolation between different or neighboured devices on the same chip can be made be arranging isolating trenches and pn junctions there between.

[0002] Isolation of semiconductor devices is also required to minimize leakage currents, to suppress the latch-up with adjacent devices and for minimizing the die size of integrated circuits. Buried layers of dopants are another method of providing device isolation.

[0003] In US 6,890,833 B2 one or more isolating trenches arranged in parallel and filled with a dielectric are disclosed. The trenches may have a depth of 600 nm e.g. and an aspect ratio of 6:1. For this type of trench a shallow-trench isolation (STI) can be considered.

[0004] Published U.S. patent application US2005/0179111 A1 discloses a CMOS device being isolated against the substrate and neighboured by a buried layer and by a DTI (deep trench isolation). The STI has a depth of typically 5 µm.

[0005] Such isolation is traditionally very important (although not limited) to HV devices such as HV transistors operating at Voltages higher than 5V and a power of > 1W.

[0006] Traditionally HV (High Voltage) semiconductor manufacturing processes and HV transistors have either been developed without buried layers (called HV CMOS) or with buried layer (BCD ... Bipolar CMOS DMOS). Currently there is no truly modular extension of a HV CMOS process towards a BCD process available. It is an object of this invention to provide such a truly modular extension.

[0007] Key requirement for such a modular extension is that LDMOS High Voltage transistors in the HV CMOS process can be operated also when they are surrounded by an optional buried layer (BL). E.g. for LDMOS (Lateral Double diffused MOS) devices , which have a reverse polarity capability below - 50V in a lowly p-doped epitaxial layer, this requires a depth of > 20µm from the surface.

[0008] In order to contact the buried layer at such large depths a doped "deep sinker contact" is required. Reducing the isolation distance requires another trench that is deeper than the "deep sinker contact" trench.

[0009] It is therefore an object of this invention to provide such a modular extension of a HVCMOS process while keeping the additional process complexity and thus the manufacturing cost as low as possible.

[0010] Another object of this invention is to provide a low cost implementation of a Deep Trench Isolation in combination with a Buried Layer.

[0011] These and other objects of the invention are obtained by device according to claim 1. Advantageous embodiments of the invention as well as a process for manufacturing the device are subject of further claims.

[0012] A semiconductor device is given, having a semiconductor body. Near the surface of the semiconductor body at least two different transistors working at different potentials are arranged. A first and a second trench are produced in the semiconductor body having, relative to the surface of the semiconductor body, a first depth d0 and a second depth d2 respectively, where d2 > d0. Both trenches are filled with a dielectric material. The first trench is surrounded by a doped region.

[0013] The semiconductor body may comprise silicon and silicon/ germanium or another semiconductor material used for semiconductor devices comprising transistors. Different types of transistors may be present in the semiconductor device, for example CMOS, NMOS, PMOS, LDMOS, BICMOS or bipolar transistors. The transistors my be operated at different voltages reaching from low voltages up to high and very high voltages, for example up to 200V. The given new trench arrangement in the semiconductor device allows to integrate all those different devices into one production sequence without the need to change the structure of any device structure.

[0014] The semiconductor device may comprise a plurality of each type of trenches, which are arranged according to the respective structures of the two different transistors. The second trench is used as a deep trench isolation DTI between different transistors which are to be isolated against each other. The second depth of the DTI may be selected in respect to the voltage applied to one of the transistors to be isolated against each other. Proper depths may be selected between 5 and 25µm but may also have a lower or even greater depth. HV transistors operating with high voltages reaching for example from 5V up to 200V need the deepest DTI.

[0015] The first trenches are surrounded by a doped region and provide a electrically conducting region around the trench which may be used to provide a current conducting path one of the transistors, a contacting plug to connect a buried layer to a terminal on the surface of the semiconductor body, or which may simply be used to provide an isolating pn junction to isolate a transistor against an adjacent device.

[0016] Both kind of trenches can be produced in parallel in the same step and both are filled with a dielectric. Thus, the manufacturing of the is easier.

[0017] A buried layer may be produced in the surface of a semiconductor substrate by implantation for example. Upon the buried layer an epitaxial layer is produced. The thickness of this layer corresponds to the depth of the buried layer. Thus, the depth of the first trench used to contact the buried layer has to correspond to the depth of the buried layer. Usually the buried layer extends only over a given area of the whole device where an enhanced electrical isolation is needed. The buried layer may comprise a plurality of non-connected areas, each under a transistor or another device structure to be isolated.

[0018] The depth of the second trenches used as DTI extends over the depth of the buried layer. Second trenches may completely surround a device structure which may comprise one or more transistors.

[0019] The device may comprise a LDMOS transistor using the doped region surrounding the first trench as a drift zone of the transistor. Thus, the first trench may be arranged between the gate and the drain of a LDMOS transistor, the drain contact arranged on top of the doped region around the first trench. In this arrangement the depth of the first trench can be properly selected to correspond to a desired drift zone length which usually corresponds to the operating voltage of the LDMOS transistor. A vertically arranged drift zone helps to reduce the chip area necessary for the LDMOS structure relative to a horizontally oriented drift zone. In this case the conductivity type of the dopant in the doped region of the first trench corresponds to the channel type of the LDMOS. Thus, a n-conducting LDMOS needs a n-doped region at the first trench. Vice versa for a p-conducting LDMOS a respective p-dopant is needed.

[0020] The same first trench can be used to provide a drift zone and a contact plug to a buried layer at the same time.

[0021] If the first trench is used for isolating a device structure the conductivity type of the dopant in the doped region has to be opposite to the channel type thereby providing an isolating vertically oriented pn junction.

[0022] The same first trench can be used to provide a drift zone and a contact plug to a buried layer at the same time. On the other hand a first trench can be used to provide an isolating pn-junction between two device structures and a contact plug to a buried layer at the same time.

[0023] The buried layer may be contacted by a special arrangement of three trenches of the same depth arranged in parallel. If the middle one is a first trench surrounded by a doped region the both outer third trenches function as a diffusion stop for the dopant in the doped region around the middle/first trench. As a result the dopant concentration is not as much reduced compared to a first trench without a diffusion stopping trench and the conductivity in this region remains high thereby reducing the losses. On the other hand, a diffusion stop limits and reduces the necessary space for the doped region. Thus, the distance between the contacting trench and the adjacent device structure may be reduced.

[0024] In another embodiment, a further trench filled with a conducting material may be used to provide a contact to the buried layer. This trench may be etched in the same process step together with the other mentioned trenches present in the semiconductor device.

[0025] For manufacturing trenches with different depths parallel in same step a specially structured etch mask is used. First a hard mask which may comprise a sequence of different layers comprising oxide and nitride layers is deposited to the entire surface of the semiconductor body. Then, elongated openings for all the necessary trenches are structured in the hard mask.

[0026] An RIE (reactive ion etch) step follows. By using the RIE lag effect a properly selected width of the opening in the hard mask leads to a corresponding trench depths. Thus, different trenches having different depth may be produced by using openings with different widths. The wider the width of the opening the deeper the trench is etched thereby using only one etch step for all different trenches.

[0027] In the next step, a dopant is introduced into the sidewalls of the first trenches. Thereafter the sidewalls are lined with a dielectric. A thermal oxide as first layer of the liner is preferred. At least, the trenches may be filled with a dielectric material which may be selected to fill deep trenches having a high aspect ratio greater than 5:1 or exceeding 10:1. A doped glass material doped with phosphorous or boron is preferred for this purpose as it flows at relative low temperatures filling the trenches without voids.

[0028] Alternatively a sandwich layer of doped and undoped glass is used to fill the trenches to reduce the cost of the deposition process and improve fill properties.

[0029] In a later step which may be subsequent or which may be made together with a later manufacturing step producing a necessary thermal budget, the dopant may be diffused deeper into the semiconductor body to produce the doped region around the first trenches.

[0030] Introducing the dopant into the sidewalls of the first trench comprises a step of ion implanting the dopant into the sidewalls. Alternatively the dopant may be introduced via a dopant source layer deposited at the sidewalls and a step of diffusing out the dopant by a thermal treatment. Subsequently the dopant source layer may be removed, for example by wet etching.

[0031] Introducing the dopant into the sidewalls may comprise two similar steps differing by different dopants to be introduced. Doing this two different first trenches having a doped region of different conductivity type can be produced. In the first step, those first trenches which are to be doped at their sidewalls with a second dopant in a second step are covered with a resist mask shielding those trenches from implantation with the first dopant in the first step.

[0032] Using a dopant source layer in the first step needs structuring the source layer to remain only in the areas to be doped. Vice versa the second dopant can be introduced.

[0033] The fourth trenches used for providing a special contact plug for the buried layer may be produced as follows:

• In step B) fourth openings are structured in the hard mask together with the openings for first and second trenches.
• In step C) fourth trenches are etched at the same time and together with the first and second trenches.
• The fourth openings are lined with a dielectric,
• The lining is removed at a bottom portion of the fourth trenches.
• The fourth trenches are filled with a conducting material.

[0034] For the conducting material doped poly may be used.

[0035] All trench fillings can be made by depositing a filling conducting or dielectric material to the entire surface of the device in a step covering process. The height of the this layer is selected to be at least half time the width of the widest trench to be filled. Subsequently the filling material can be remove from all device areas other than trenches by back etching, CMP or another proper process.

[0036] In the following aspects, features and advantages of the present invention will be discussed with reference to the accompanying drawings depicting preferred embodiments of the invention. The figures are only schematics and not drawn to scale.

Fig. 1 shows several steps of the formation of isolation trenches via ion implantation

Fig.2 shows several steps of the formation of isolation trenches via of deposition of doped glass

Fig.3 shows an arrangement of all 3 types of trenches that can be provided by the current invention

Fig.4 shows an transistor with vertical drift region and DTI

Fig.5 shows another LDMOS transistor with vertical drift region, DTI and a buried layer

Fig.6 shows an LDMOS structure with DTI, a first trench forming a sinker contact and a buried layer

Fig.7 shows another LDMOS structure with DTI, a first trench forming a sinker contact and buried layer

Fig.8 shows a LDMOS structure with trenches, which help to prevent out-diffusion of a trench sinker.

Fig.9 shows a simulation of leakage currents of the embodiment of figure 6

Fig.10 shows a simulation of leakage currents of that embodiment with an applied voltage of reverse polarity

Fig.11 shows the positive effect of a deep trench isolation.

[0037] Fig. 1 shows a first embodiment of this invention which enables the formation of different trenches preferably filled with oxide or other suitable dielectrics that can optionally be surrounded by a p-doped or an n-doped region at the trench - silicon interface.

[0038] The process of trench formation starts with depositing hard mask onto a semiconductor body e.g. a silicon wafer and structuring said mask to provide elongated openings according to the trenches to be produced. The width of the trenches is selected in accordance with the desired depth of the trenches. The wider the opening the deeper the trench. Etch of trenches is performed in a well known one step RIE process for all trenches at the same time.

[0039] After removing the hard mask an implantation mask IM is deposited to the entire surface, for example 5nm LPCVD silicon nitride. A first resist mask RM1 is applied thereon and structured to expose the implantation mask IM in the region of a first trench T1 and to cover the regions of a second trench T2 and a third trench T3. All the trenches T1 to T3 are arranged in different regions of the same semiconductor body as shown in figure 1A.

[0040] In the next step the exposed implantation mask is etched, e.g. by wet etch using phosphoric acid. Figure 1 B shows the device after stripping the resist mask RM1. In the region of a first trench T1 the implantation mask IM is removed.

[0041] Then, a dopant is introduced into exposed trench sidewalls of first trench T1 either by small angle Ion Implantation (I/I), Plasma doping (PLAD) or PIII (Plasma Immersion Ion Implantation), or Gas Phase Doping to produce a first implanted region IR1. The dopant is of an first conductivity type.

[0042] In the next step the first implantation mask IM1 is stripped and a second implantation mask IM2 is deposited and structured as shown in figure 1D. Now the region of a third trench T3 is exposed while the other trenches T1, T2 remain covered by the second implantation mask IM2. The already explained steps according to figures 1B and 1C are repeated to result in an arrangement as shown in figure 1E. In the sidewalls of the third trench T3 an implanted region IR3 doped with a dopant of an second conductivity type is produced.

[0043] Then the sidewalls are sealed by thermal oxidation (e.g. 5-10nm SiO₂). In a subsequent thermal anneal the dopants are introduced deeper into the trench sidewall to produce the doped regions DR1 and DR3 surrounding the first and third trench T1 and T3. This does not have to be a separate process step but can also occur much later in the process utilizing other thermal anneals such as a well drive-in needed for the formation of HV or LV CMOS wells.

[0044] The remaining cavity in the trenches may be filled by depositing a dielectric filling DF as shown in figure 1F. The layer extending over the surface of the semiconductor surface may be removed.

[0045] Fig.2 shows, in a second embodiment of the current invention another way to produce different doped regions around the first trenches. Formation of isolating first trenches with optional surrounding p-doped or n-doped region is enabled by deposition of a first dopant source layer DS1 for example doped glass (e.g. BSG for realizing p-doped regions; e.g. ASG or PSG for providing n-doped regions). After deposition the doped glass is structured with a third resist mask RM3 where exposed regions of the dopant source layer DS1 are removed selectively that only those trenches remain covered which are sought to be doped. In figure 2A the area of first trench T1 is covered with resist RM3. The glass DS1 can be removed by using isotropic wet or dry chemical etching.

[0046] After structuring of the doped glass layer DS1 a second dopant source layer DS2 containing a second dopant deposited and structured with a fourth resist mask RM4. Figure 2B shows the situation at this point where only the area of third trench T3 is covered with resist RM4. Subsequently exposed second dopant source layer is removed by etching and a cap layer CL (e.g. LPCVD oxide or nitride) is deposited. Then a thermal anneal is performed to drive the dopants from the doped glass into the silicon region surrounding the trenches (see fig.2C). A doped region DR1 forms around the first trench T1 and another doped region DR2 of opposite conductivity type (e.g. a p-doped region) forms around the third trench T3. Alternatively other anneals such as a well drive-in which are part of the subsequent CMOS process flow can be used instead.

[0047] Fig.3 shows three kinds of different trenches that can be provided by the current invention, after trench fill and recess. Trenches can simultaneously be provided in various depths by varying trench layout parameters such as length and width (not shown). They can be surrounded without (T2) or with a doped region of a first (T1) or second conductivity type (T3).

[0048] Fig. 4 shows an LDMOS transistor with vertical drift region and DTI which may be manufactured using trenches produced as described above. The LDMOS transistor comprises a p-doped body SB, n-doped well NW for drain D, p-doped well NW for drain D, p-doped well PW for p-body , a gate structure G and LDD regions LDD. Between drain D and a gate a first trench T1 is located, filled with dielectric and surrounded by a n-doped region DR1 which provides a vertical drift region for the LDMOS. The LDMOS arrangement is surrounded by isolating trenches TI which may be of the first type. The p-doped regions DR2 provide a pn junction relative to the body SB isolating the LDMOS device against other devices of the same or of different type.

[0049] Fig.5 shows an LDMOS transistor with vertical drift region of depth d0, a buried layer BL at a depth d1, a sinker contact to the buried layer BL and a surrounding deep trench isolation DTI of depth d2 with d0 < d1< d2. Sinker contact and vertical drift region are provided by the same trench T1 of the first type surrounded by a n-doped region DR1. Because of the n-epi as a drift region, currents flow to the vertical direction (towards buried n-well) and are finally collected at the sinker (vertical DMOS). Between drain D at the right side of the trench T1 and gate G a field oxide region or a shallow trench isolation may be arranged providing a further lateral drift zone to support high blocking voltage.

[0050] The n-doped buried layer BL is arranged between a p-doped substrate SU and an n-doped epitaxial layer EL. A p-doped well PW provides the body of the LDMOS. Under the source a n-doped well NW is arranged.

[0051] The deep trench isolation DTI isolating the LDMOS against other adjacent devices of the same or of different type is realized by trench of the second type without surrounding doped region. DTI and first trench are manufactured in the same process as described above.

[0052] Fig.6 shows an LDMOS structure with a sinker contact to a buried layer of depth d1 and surrounding DTI of depth d2 with d1 < d2. Drain D is arranged over a deep n-well DN. Drain and source S are arranged over a n-well NW respectively. Source S can additionally function as a body contact B contacting the body formed of a p-doped epitaxial layer EL. The horizontally drift zone is formed by the n-well NW. Adjacent to the deep trench isolation DTI formed by a trench of the second type without surrounding doped region. Adjacent thereto on the right hand side a further LDMOS or any other type of transistor may follow well isolated against the LDMOS by DTI.

[0053] Figure 7 shows another embodiment of a first LDMOS structure with a sinker contact to a buried layer of depth d1 and surrounding DTI of depth d2 with d1 < d2. The buried layer is bisected by the deep trench isolation DTI. The sinker contact is formed by a first type trench T1 surrounded by a n-doped region DR1 which is in contact with the drain D. A deep n-well DN forming the drift region of the first LDMOS is also divided by the trench T1. Combined source and body contact S/B overlaps a n-well NW. Gate G arranged between source S and drain D overlaps the n-well NW, the deep n-well DN and a body area. Body SB is formed by a p-doped epitaxial layer separated from the substrate SU by the buried layer BL. Trench T1 forms a symmetry axis for a subsequent second LDMOS also using the n-doped region DR1 as drain.

[0054] Figure 8 shows another embodiment of a LDMOS structure having an isolated sinker contact zone to contact the buried layer BL under the LDMOS. The sinker contact is formed by a first type trench T1 surrounded by a n-doped region DR1. Adjacent to that region on both sides of the first type trench T1 are arranged isolating trenches T2 filled with dielectric. During manufacturing of the three trench assemble while driving out the dopant out of the side walls of the first trench T1 the both neighboured trenches work as diffusion stop, limiting the extension of the doped region DR1 and isolating same. The LDMOS realized above the buried layer BL besides the isolated contact sinker is thus well isolated. A further deep trench isolation DTI is located beside the buried layer and the contact sinker. A substrate contact Sub is arranged above a p-well PW between contact sinker and DTI. Right hand of the DTI a further transistor e.g. a LDMOS follows which may be isolated against substrate by a further buried layer or not depending on the operating voltage. The depths of the two trenches is controlled by a width W1 of first and second type trenches T1, T2 and a width W2 of DTI where W1 < W2 resulting in a depth d2 > d1.

[0055] It can be shown by simulation calculation that a buried layer and a sinker contact as shown in figure 6 for example are well isolation a transistor devices against substrate even at reversed polarity such that a punch through will not occur at voltages up to -66V where VD = VS = 66 V and the substrate is biased (ground).

[0056] Figure 9 shows calculated leakage currents at body (continuous line) and substrate contact for different positive biased voltages. The diagram makes clear that a punch through does not appear under 90V and probes for the low substrate current capabilities of the structure from Fig.6.

[0057] Figure 10 is a similar diagram for a below substrate voltage operation where VS = VD < 0V relative to substrate (VS = 0). It shows that a punch through does not appear until -60V. At positive and reverse bias voltages from zero to punch through the leakage current rises only with a very small amount.

[0058] The further deep trench isolation DTI results in a better isolation between device neighboured in the semiconductor body. A n-well for example operating with high potentials does not lead to a punch through to the transistor isolated by a buried n-layer and a sinker contact but result in a current flowing to the buried layer.

[0059] The positive effect of a deep trench isolation DTI on the isolation of a transistor is shown in figure 11. The first curve (continuous line) shows the leakage current for a device with deep trench isolation DTI compared to the leakage current for a device without DTI (doted line). The isolation in the first case is improved relative to the second case for more than 90 V.

[0060] As the invention has been explained by few embodiments only it is not restricted to same. It is clear that the inventive trench combinations can be used to isolate any kind of transistor device realized in a given semiconductor body of any semiconductor material or any conductivity type. All dopings in the examples can be reversed without deviating from the invention and its superior isolating properties.

Claims

1. A semiconductor device, comprising

- a semiconductor body (SB)

- at least two different transistors which are arranged in the semiconductor body and are working at different potentials

- a first trench (T1) produced in the semiconductor body having a first depth d0 relative to the surface of the semiconductor body

- a second trench (T2) produced in the semiconductor body having a depth d2, where d2 > d0

- both trenches filled with a dielectric material

- the first trench being surrounded by a doped region (DR).

2. The semiconductor device of claim 1,
wherein the semiconductor body (SB) comprises a semiconductor substrate (SU) and an epitaxial layer arranged thereon, and a doped buried layer (BL), having depth d1 arranged between the substrate and the epitaxial layer wherein d0 < d1 < d2;
wherein the depth d0 of the first trench (T1) corresponds to the depth of the buried layer and wherein the doped region surrounding the first trench provides a contacting connection of the surface of the epitaxial layer to the buried layer.

3. The semiconductor device of claim 1 or 2,
wherein the second trench provides a deep trench isolation (DTI) between a first and a second transistor, at least one of the transistors being a HV transistor.

4. The semiconductor device of one of claims 1 - 3, comprising a LDMOS transistor using the doped region surrounding the first trench as a drift zone of the transistor.

5. The semiconductor device of one of claims 1 - 4, comprising a plurality of first trenches (T1), at least one of the plurality being surrounded by a doped region (DR) of a conductivity type different to that conductivity type of another one of the plurality of first trenches.

6. The semiconductor device of claims 4 or 5, comprising a LDMOS transistor of a first channel type which uses the doped region (DR1) surrounding the first trench (T1) as a drift zone of the transistor,
a further first trench, surrounded by a doped region (DR2) of a conductivity type opposite to the channel type, the further first trench arranged adjacent to the LDMOS transistor isolating same against a neighboured transistor of the same or a different type.

7. The semiconductor device of one of claims 4 - 6, comprising a substrate (SU), a buried layer (BL), an epitaxial layer and a LDMOS transistor, wherein the first trench (T1) providing the drift zone of the LDMOS transistor provides at the same time a contacting connection of the surface of the epitaxial layer and the buried layer.

8. The semiconductor device of one of claims 1 - 7, comprising two third trenches, filled with a dielectric, arranged on both sides of a first trench and having a depth d0, the two third trenches confining the doped region of the first trench arranged there between.

9. The semiconductor device of one of claims 1 - 7, comprising a fourth trench, lined with a dielectric, filled with a conducting material, and having a depth d0, the conducting material providing a contacting connection of the surface of the epitaxial layer and the buried layer.

10. The semiconductor device of one of claims 1 - 9,
wherein
the semiconductor body comprises a buried layer having a limited area and being arranged in a depth d1 where d1 < d2, a transistor is arranged in the semiconductor body above the buried layer
the first trench is arranged to provides the contacting connection of the surface of the epitaxial layer and an outward edge of the buried layer
the second trench of depth d2 being arranged besides the buried layer,
a substrate contact being arranged at the surface of the semiconductor body between the first and second trench.

11. A method of manufacturing first and second trenches (T1,T2) in a semiconductor body (SB),

A) providing a semiconductor body

B) depositing and structuring a hard mask on a surface the semiconductor body such that first and second elongated openings for first and second trenches are produced, having a first and a second width W0 and W2 where W0 < W2

C) etching first and second trenches (T1,T2) of different depths d0 and d2 in a common RIE etching step, where d0 < d2

D) introducing a dopant into the sidewalls at least one of the first trenches

E) covering the sidewalls of the first and second trenches with a dielectric

F) driving the dopant into a region surrounding said at least one first trench.

12. The method of claim 11,
wherein introducing the dopant into the sidewalls of the first trench comprises ion implanting of the dopant.

13. The method of claim 11,
wherein introducing the dopant into the sidewalls of the first trench comprises

- depositing a dopant source layer to the entire surface of the semiconductor body and the sidewalls of the trenches,

- structuring same such that the remaining portion of the dopant source layer covers the sidewalls and an adjacent region of the first trench, and

- driving the dopant out of the source layer into said sidewalls, and removing the source layer.

14. The method of claim 12 or 13,
wherein step D) is repeated to introduce a dopant of opposite conductivity type into the sidewalls of another trench of the first type.

15. The method of one of claims 11 - 14,
comprising a step G) to be performed after step F) or between step E) and F)

G) filling first and second trenches with a dielectric.

16. The method of one of claims 11 - 15, wherein

- step B) comprises structuring of fourth openings

- step C) comprises etching of fourth trenches at the same time with the first and second trenches respectively,

- the fourth openings are lined with a dielectric,

- the lining is removed at a bottom portion of the fourth trenches

- the fourth trenches are filled with a conducting material.

17. The method of claims 16,
wherein the fourth trenches are filled with doped polysilicon and used to provide a contact of the buried layer to the surface of the semiconductor body.

18. The method of one of claims 11 - 17, wherein
wherein in Step B) openings of at least three different widths are produced thereby resulting after etching through said openings in three trenches having three different depths.

Drawing